The present invention relates to integrated circuit structures and fabrication methods, and especially to the deposition of tungsten.
As device sizes continue to shrink, metals other than aluminum are being studied for use in the formation of interconnections, such as contacts/vias, metal suicides, etc. Tungsten, which can be deposited by chemical vapor deposition, has low resistance and stress, excellent step coverage, and a thermal expansion coefficient which closely matches silicon. However, one of the problems in the deposition of CVD tungsten is its low adhesion to dielectrics, so that a deposited adhesion layer, e.g. TiN, is generally used under tungsten. See, e.g., Wolf et al., Silicon Processing for the VLSI Era, Volume 1xe2x80x94Process Technology (1986), which is hereby incorporated by reference.
In addition to providing adhesion, the adhesion layer also reduces the deposition incubation time. However, as devices dimensions shrink, the requirements of an adhesion layer make it difficult to fill small contacts or vias with high aspect ratios, using the CVD W process, as the adhesive layer reduces the available space. Additionally, for applications using tungsten-capped polysilicon as a gate electrode, the adhesion layer adds process steps and increases cost.
The present application discloses several methods of forming a very thin adhesion layer under the CVD tungsten, or alternatively, of depositing CVD tungsten on silicon and dielectrics without an adhesion layer at all. In some embodiments, a very thin layer of silicon is formed on the surface of a silicon or dielectric layer, then nitrided in situ to form a very thin layer of silicon nitride. In other embodiments, a thin layer of tungsten is deposited over polysilicon, then nitrided in situ to form WNx. Either of these nitrided layers can provide an adhesion layer without unnecessary bulk. A further embodiment forms a thin layer of silicon, which is then exposed to WF6, causing the silicon to be replaced by a thin nucleation layer of tungsten. Further deposition of tungsten uses a WF6/H2 chemistry.
Advantages of the disclosed methods and structures follow. Not every advantage may be applicable to every embodiment:
easy to manufacture;
does not require a separate chamber for forming nitride;
takes up less space in contact or via;
effective in the formation of CVD tungsten on dielectric;
effective in preventing reaction between tungsten and silicon;
compatible with current process flow;
produces an effective diffusion barrier.